What makes apple trees tick?

Lakso used “full canopy balloon chambers” to measure carbon dioxide removal from air to study whole tree photosynthesis and later used them to study water stress on trees. (Courtesy of Dr. Alan Lakso)

During his 40 years at Cornell University, Dr. Alan Lakso devised some oddly clever ways to figure out how fruit trees do the things they do.

For example, he used laser beams as artificial sunbeams to observe which kinds of leaves in apple trees capture the sunlight. Earlier studies had shown that early fruit growth, around thinning time, was supported by spur leaves, not extension shoot leaves. The work with the laser beams showed that more productive orchards captured much more of the sunlight with spurs, while orchards with poor productivity captured the light with extension shoots.

Lakso used full canopy balloon chambers, which involves enclosing trees in clear plastic bubbles with air flowing through. By measuring the carbon dioxide removed from the air, he could measure whole tree photosynthesis that provides carbohydrates for the tree to grow shoots, roots, and fruit. With these chambers he was able to determine how European red mites and summer pruning interacted with other factors to affect fruit sizing.

These tools taught him many things about trees and how they function. That information has been shared over the years with other researchers and growers, and it has affected how growers plant their orchards, prune their trees, and thin their fruit.
As mentioned, leaves just below the growing shoot tips send all the energy they collect to the tips to make them grow. They’re not important contributors to early fruit production until shoots stop growing. Spur leaves, on the other hand, send all their energy to fruit growing on the spurs.

From that, he concluded that using the plant growth regulator Apogee (prohexadione calcium) to shorten shoots should allow more carbohydrate to go to early fruit growth, which leads to the need for harder thinning.

He also found that shaded leaves not only fail to contribute to the fruit crop once they lose that ability to photosynthesize, but they never get it back even if re-exposed to light.

From that, he concluded that summer pruning, while it exposes fruit to light and improves fruit color, may reduce the tree’s overall ability to photosynthesize since newly exposed leaves don’t help out as much as expected.

“Midsummer pruning can reduce tree function and light interception,” he said. “This may lead to poor fruit sizing.”

Dr. Alan Lakso spoke about the work of his career during the Cornell University In-Depth Fruit School in Geneva, New York, in March. He was one of six tree fruit ­physiologists called together, as their research careers are nearing their end, to talk about what they had learned.

Light distribution

Apple yield is controlled by fruit numbers, which are determined early in the season, Lakso said. “At that time, fruit is supported by spur leaves, so orchard yield depends on light interception by spurs.”

Later in the season, shoot leaves contribute more, but early on, they contribute virtually nothing.

From that fundamental fact, the shape of modern fruit orchards has emerged. Growers know they have to prune to assure that light can penetrate to all the spurs for the first month after bloom. Trees need to have adequate gaps in the canopy. Trees should be oriented in north-south rows to minimize shading on the north side. Trees should be slender so light can penetrate the canopy.

Trees also need to be tall if maximum yields are to be achieved. Lakso’s fundamental work contributed greatly to the tall spindle system developed at Cornell by Dr. ­Terence Robinson and Steve Hoying.

Fruit set and drop

Lakso’s physiological research also laid important groundwork for the Cornell MaluSim carbohydrate model now being used by growers in New York, Michigan, Pennsylvania, and other eastern fruit-growing states. Robinson has been the lead researcher adapting the model for practical use, which growers can access through the Northeast Regional Climate Center.
The model integrates how weather affects the carbohydrate supply versus demand. This helps predict how effective fruit thinning chemicals will be, depending on upcoming weather conditions. The usefulness of the model depends on having accurate predictions of temperature and cloudiness, and the New York researchers have worked closely with the Northeast Weather Network to help growers have better predictions of weather on their farms.

The model is based upon basic principles of carbon production and partitioning that Lakso sought to understand. If the carbon supply is limiting, then the relative strengths of the sinks of different tree organs are used to partition carbon, he said.
Trees partition carbon to leaves first, then to fruit, and finally to roots and tree wood. “Fruit drop if there are too many competing fruit and if they can not continually grow. When early season fruit growth is reduced for any reason, drop occurs when fruit have too low growth rates. Low light causes fruit drop,” Lakso said, especially when it occurs with high temperatures, especially at night.

“Weather effects on drop and thinning response seem to be consistent with carbohydrate supply/demand ­balance,” he said.

Pests and pruning

Over the years, growers have fought their battles against European red mite and foliar diseases that reduce the photosynthetic ability of trees to size fruit.

Lakso did experiments in collaboration with entomologists to determine how much effect they had. He found that whole tree photosynthesis, and thus ­carbohydrate supply, is reduced by leaf damage, by shading of leaves, and by summer pruning to remove some foliage to improve fruit color. The effects on fruit size, however, depend on the level of crop (and carbohydrate demand), with heavily cropping trees being especially sensitive to reductions in carbohydrate supply.

“Reduced tree photosynthesis can affect return bloom and future fruit set as well,” he said, and also can lead to weak flowers. “Ideally we need to prune to have open canopies so we don’t need to summer prune so much,” he said.

Water stress

Lakso also spent many research hours studying water stress on apples and grapes.

“Woody plants are often on variable soils and generally have large, deep, but relatively low-density root systems, unlike annuals,” he said. “The water status of trees and vines strongly depends on the weather as well as the soil. It is very dynamic with changes in the weather.”

Last year, Lakso, working with chemical engineer Abe Stroock and graduate student Vinay Pagay, developed a microsensor than can be embedded in a tree or vine where it can measure water stress directly. Like miniaturized tensiometers that cost around $75, the microsensors are low cost because they are chips that can be easily manufactured. They have been tested in the lab and are very accurate over a large range.

They were described in an article in Good Fruit Grower in the December 2013 issue.

Lakso said the microsensor was a dream come true. He’s been working to develop such a thing for more than 15 years, and, on the eve of his retirement, it’s within reach. The device is being patented.